This is Westminster code. What it means is that the Lib Dems and Tories are competing for the support of people who once thought, but can believe no longer, that Labour is the natural home for those who agitate for a fairer society.

Both Mr Clegg and Mr Cameron write about the environment, civil liberties, social mobility and the need to shift power away from a centralised state. The difference is that the Lib Dem leader's case is sustained over a thoughtful pamphlet that draws on more than a century of his party's history, while the Conservative leader's article is a pre-conference tactic, an attempt to claim joint credit for positions the Lib Dems developed while the Tories were still nursing authoritarian reaction.

It is true that the Lib Dem and Tory leaders have a lot in common, in matters of style and political instinct. What is intriguing is that Mr Cameron sees this as something to shout about, while Mr Clegg fiercely resists claims of any likeness. He opened his party conference yesterday with an attack on his Tory counterpart as the "conman of British politics".

Mr Cameron's pitch to Lib Dem voters reveals an underlying insecurity. His party has a commanding poll lead. He is treated with deference by much of the media as a PM-in-waiting; Mr Clegg struggles to be heard. Why should the Conservatives feel the need to drape themselves in Lib Dem colours when they may simply ignore the third party?

The answer is that the Tories are not winning as much as Labour is losing. The poll margins are still narrow enough to make a hung parliament a real possibility. Lib Dem MPs are a Tory wobble away from holding the balance of power.

Mr Cameron hopes that floating voters will feel relaxed enough about the prospect of a Tory government to skip the Lib Dems and go blue to make sure Labour are evicted.

Mr Clegg, by contrast, needs to revive in people's minds the suspicion that the Conservatives do not care enough about social protection to make it a priority in an age of budget austerity. He needs voters who are fed up with Labour to see the Lib Dems as a vital brake on a triumphalist, ultra-Thatcherite Tory revival.

That is a plausible and necessary proposition going into the next election. All of the parties have struggled to tune their message to turbulent economic signals, but the Lib Dems have fared better than the Tories. Last year, they backed fiscal stimulus when Mr Cameron preached dangerous inaction. Now they name areas for budget restraint while the Tories keep the targets of future cuts secret. Lib Dems talk openly about using the tax system to make society fairer. Tory tax policy is a mystery apart from favours for wealthy legatees and married couples.

There was a point when it seemed as if Mr Cameron could raid the Lib Dem policy wardrobe whenever he wanted – a green idea here, some civil liberties there – and get away with it. But that was in rosier economic times. Now Mr Clegg can call his bluff: let the Tories claim the increasingly vague "progressive" label; talk instead about equality and fair distribution of wealth; see if Mr Cameron dares follow.